To increase a breakdown voltage, a semiconductor device having a region (i.e., a drift region) including a low impurity concentration is disclosed.
By using the drift region, the breakdown voltage of the device is increased. However, an on-state resistance of the device is also increased. Accordingly, it is required to increase the breakdown voltage together with reducing the on-state resistance. In view of this requirement, a super junction structure is proposed. The super junction structure has a first region including a first conductive type impurity and a second region including a second conductive type impurity. The first region extends along with a direction connecting between a pair of main electrodes. The second region extends in parallel to the first region. The first conductive type in the first region is equal to a conductive type of a carrier moving between the main electrodes. In the super junction structure, a combination of the first and second regions is repeated in the plane, on which the main electrodes expand. In the plane, the first region is sandwiched between two of the second regions, and the second region is sandwiched between two of the first regions.
FIG. 11 shows an on-state resistance of a semiconductor device disclosed in ISPSD Proceedings, 2005, pages 35-38. A vertical axis represents the on-state resistance, and a horizontal axis represents a temperature of the device. A curve 62 in FIG. 11 represents the on-state resistance in a case where a cell pitch of a semiconductor element is 6 μm, and a curve 64 represents the on-state resistance in a case where a cell pitch of a semiconductor element is 6.2 μm. The on-state resistance of the curve 64 is smaller than that of the curve 62. Thus, to reduce the on-state resistance, it is preferred that the width of the first region sandwiched between the second regions becomes larger.
However, as shown in FIG. 11, the on-state resistance of the device is varied together with temperature change. To reduce the on-state resistance, the device is cooled so that an operation temperature of the device becomes constant.
Specifically, when the device switches electric power, the device generates heat. In general, it is preferred that the operation temperature of the device is disposed in a range between a room temperature (i.e., 27° C.) and 150° C. In the above device having the drift region, when the on-state resistance at 27° C. is defined as R1, and the on-state resistance at 150° C. is defined as R2, a ratio between R2 and R1 is equal to or larger than 18. When the device having the ratio of R2/R1 equal to or larger than 1.8 is used for a power switching device, high performance cooling equipment is required to cool the device and to maintain the operation temperature in a narrow range such as in a range between 27° C. and 150° C. The cooling equipment has a heat radiation plate and a coolant water passage, which have complicated design.
Thus, it is required to decrease a change rate of the on-state resistance with respect to temperature. Specifically, it is required to reduce the ratio of R2/R1 equal to or smaller than 1.8. In this case, required cooling performance of cooling equipment for the device can be reduced. Thus, the cooling equipment is minimized and simplified, so that the dimensions of the device are reduced.